Alkyne Metathesis in Organic Synthesis

While alkyne metathesis is not going to displace alkene metathesis as a
synthetic method, it is a complementary approach that can offer advantages.

Catalysts for alkyne metathesis are still under active development. A recent
paper by Karol Grela of the Polish Academy of Sciences in Warsaw (J. Org.
Chem.2004, 69, 7748.
)
provides
an detailed overview of the options. The choice is between sensitive preformed
catalysts that provide high turnover but require more exacting organometallic
techniques, or, alternatively, in situ catalysts that require higher temperature
and longer reaction times, but are less expensive and less technically
challenging to prepare. For the latter, the inexpensive Mo(CO)6 has
been the precursor of choice, with an added phenol ligand. In this paper,
Professor Grela and co-workers optimized the supporting phenol, finding that
2-fluorophenol was the most effective. Dimerizations with this catalyst system
require no special precautions - indeed, they can be run open to the air. Both
cyclodimerization (1->2) and cross metathesis (3->4)
proceeded efficiently.

The more widely used alkene metathesis is deficient in that the alkenes so
prepared are often mixtures of geometric isomers. Alkyne metathesis offered what
promised to be a general solution to this problem. Hydrogenation of the alkyne
products to the Z-alkene was straightforward. Reduction of the isolated
alkyne to the E-alkene was not so obvious. Alois Fürstner of the
Max-Planck-Institut, Mülheim, has surveyed (Tetrahedron2004,
60, 7315.
)
several approaches, then optimized the most promising, Ru-mediated
hydrosilylation followed by protodesilylation. The procedure worked equally well
to construct E,E-dienes. This promises to be a mild and general solution
to the long-standing challenge of E-reduction of an alkyne in the
presence of other functional groups.

In each of the alkyne metatheses outlined here, the byproduct is the volatile
2-butyne. The alkyne metathesis can only be carried out on internal alkynes,
since the metathesis catalysts cyclotrimerize terminal alkynes such as 11
to benzene derivatives. In this context, it may prove useful that
readily-available terminal alkynes such as 14 are easily isomerized
specifically to methyl alkynes such as 15 (Tetrahedron Lett.
1990, 31, 5843.
).